Ceramic materials are often utilized in applications requiring the joining of such materials to either other ceramic materials or to metal components. In the electronics industry, vacuum tube production has utilized such ceramic to metal joints, and currently cesium vapor arc lamps utilize brazed ceramic to metal end cap seals. In the automotive industry, ceramic is utilized in wear pads which are joined to metal parts to prolong the service life of such parts.
In the past, a two step technique for brazing ceramic materials to metal components was utilized. A molybdenum/manganese paint was first applied to the ceramic substrate to be joined. The molybdenum/manganese painted surface would then be plated with nickel. The metallized surface of the ceramic would then be brazed to the desired metal component with a suitable brazing compound. The process was highly time consuming and required metallization at extremely high temperatures on the order of approximately 1500.degree. C. Metallization of the ceramic material was required due to the inability of brazing compounds of the past to sufficiently wet out onto the ceramic material.
In the 1940's, development of active metal brazing of ceramics began. In essence, active metal brazing allows a direct brazing of a ceramic component without the need for a prior metallization step. An active component in the brazing filler chemically reacts with the ceramic to form an interfacial compound. The active component in the brazing filler may be titanium, zirconium, columbium, nobelium, vanadium or chromium. Within the interfacial compound, displaced ceramic cations from the ceramic material migrate into the filler. Most commonly, active brazing compounds are based on binary or ternary metal systems in which at least one metal is the active metal as discussed above.
U.S. Pat. No. 2,570,248 discloses a mixture of titanium hydride (TiH.sub.2) and a powdered solder, such as a copper solder for in bonding "non-metallic bodies" such as ceramics, porcelains, glasses, carbons, and diamonds, to other non-metallic bodies or to metal components.
The titanium component of this mixture is the active metal component that allows direct brazing to the ceramic material. When utilizing a ceramic material such as Al.sub.2 O.sub.3, the reaction that takes place between an active brazing filler component such as titanium can be represented as: EQU 3(Ti)+2Al.sub.2 O.sub.3 .fwdarw.3TiO.sub.2 +4(Al)
The aluminum cations are free to move into the braze filler metal as the titanium oxide in the interfacial compound provides the wetting of the ceramic surface which had previously been accomplished with the two step metallization process described before.
The process of this patent suffers from certain inherent deficiencies. The coating material includes a mixture of titanium hydride and a copper, silver or gold based solder. The coating is applied by a painting technique wherein irregularities in coating thickness and coverage often occur. These irregularities lead to localized weakness in joint strength as well as to evaporation of the solder. Furthermore, when such coating materials are utilized to join ceramics in a vacuum, the hydrogen liberated from the titanium hydride must be removed requiring additional pumping off of such gas.
U.S. Pat. No. 2,857,663 discloses a technique for bonding ceramic materials to other ceramic materials or to metals wherein a shim, often a thin foil, is placed between the ceramic and the component to which it is to be joined. The foil is comprised of at least one metal of the titanium group (the active metal) and an alloying metal such as copper, nickel, molybdenum, platinum, cobalt, chromium or iron. The alloying metal is selected so as to form a eutectic alloy at a temperature below the melting temperature of any one of the alloying metals. The titanium group includes metals in group IVb of the standard periodic table, such as titanium, zirconium, hafnium, and thorium.
The foil technique utilized in this patent allowed a uniform application of alloy and active metal between the components to be joined. Additionally, since the hydride form of titanium was not utilized, there would be no hydrogen gas to remove from the reaction atmosphere.
In an effort to simplify the art of joining ceramic components to other parts, brazing alloys have been formulated which incorporate up to about 2.5% titanium in a eutectic mixture of copper and silver. As discussed above, it is the titanium group metal that is active in wetting the surface of the ceramic. There must be provided within the brazing filler a sufficient quantity of titanium to react with the ceramic so as to form a substantial interfacial layer. Yet, in alloys presently available, there is a functional limit upon the amount of titanium group active metal which may be utilized.
Brazing alloys are subject to a phenomenon known as blushing. As a brazing alloy is heated, surface flow of the filler metal may occur. As the surface flow of the filler metal increases, depletion of the active metal component occurs. This causes a depletion in the amount of active metal available for reaction with the ceramic so as to form the interfacial compound. A decrease in the interfacial compound results in a weaker bond between ceramic or ceramic/metal components joined by such brazing alloys. Brazing materials presently available containing titanium or titanium in an amount above about 2.5% lead to increased blushing with the concomitant loss of joint strength.
INCUSIL-ABA.RTM. brazing material, a product of the Wesco Division of GTE Products Corporation, is typical of the brazing alloys presently available utilizing titanium as an alloy component. The recommended brazing temperature of this alloy is from 715.degree.-740.degree. C. After the brazed components are heated to this temperature, they are cooled at a controlled rate. It is well known that the ceramics have a coefficient of thermal expansion substantially lower than that of metallic materials. One of the major difficulties in attaining a brazed joint of sufficient strength between these materials concerns the high stress generated when the ceramic/metal structure cools after brazing. If this stress is not relieved, or if not redirected so as to strengthen the bond, joint failure will result.
The degree of plasticity a particular brazing alloy provides is one way in which the stress resulting from joint cooling may be alleviated. As the titanium content of a brazing alloy exceeds above about 2-3 weight percent, a hardening of the brazing alloy occurs so as to limit its capability of reducing joint structural stress as discussed above. Furthermore, it is often desirable to extrude a brazing alloy so as to form a sheet which may be cut into washers, rings or other shapes in accordance with joint configuration. The increase of titanium content, as discussed above, will increase alloy brittleness so as to make such extrusion and cutting procedures highly difficult.
The one step solid alloy brazing materials currently available are limited, in that such materials may incorporate only about 2 to 3 weight percent of a titanium group active metal, and this does not provide for optimum brazed ceramic joints. What is needed, therefore, is a one step brazing material that may be extruded into sheets and cut into various shapes wherein an optimum amount of titanium group active metal is provided in the material to ensure that a brazed joint of optimum quality is achieved.